Method of protecting magnesium and its alloys at elevated temperatures



- ,tion proceeds.

- Patented Feb. 21, 1933 UNITED STATES PATENT oFFIcE nzowmo. sans-row, or MIDLAND, moment, as'srenon r DOW cnmncn. comm,

01 MIDLAND, MICHIGAN, A. CORPORATION OF EOHIGAN METHOD OF PBOTEGTI IZNG- IAGNESIUllI AND ITS ALLOYS A 'll' ELEVA'I ED TEMPERATURES no Drawing.

The present invention relates to methods of preventing readily oxidizable metals, such as magnesium and its alloys, from combilling with constituents of the air or other the like or mixtures of the same, from combining with magnesium metal and its allo s. Another object of my'invention is to inhi it detrimental attack upon magnesium audits alloys by gases in contact with the metal'at 1 elevated temperatures such as those employed in heat treating, hot working, and foundry operations, such as melting, alloying, die casting, and the like. Other objects and advantageswill appear as the descrip- As is well known, magnesium and alloys ,of the same containing preponder'ating amounts of magnesium, that is, containing more than per cent of magnesium, react with oxygenor with nitrogen to form oxides and nitrides, respectively, when said metals are in heated condition; reactions proceed with increasing rapidity the higher the temperature. This afiinity of magnesium and its-alloys, when" in heated cond1- -tion, for oxygen or nitrogen, greatly hampers any process of handling these metals in air at an elevated temperature. As the reactions above indicated take place, the heated metal becomes coatedwith an oxidenitride-film, or, if the metal is in molten condition, these reaction products may become intermixed with the metal and cause flaws in the ca ings, {for instance, madetheref'rom.

;Heretofore various methods have been proposed to preventmagnesium and its alloys from reacting with the constituents of the air whilein highly heated condition, as

in thecase of a body of such metals in the molten state in preparation for casting. It has been proposed in United States Patent No. 906,820, when melting and pouring/magnes'rum, to heat the metal to a temperature at which it becomes pasty and then, just be- Application filed March :1,

193:. Serial F0. 600,381.

fore casting, to stir the contents of the melting pot'so as to abstract heat from the pot walls and raise the p metal to a temperature at which it will sufliciently fluid for pouring into the molds. While this' method mmimizes the amount of reaction products formed from the metal by contact with air during melting and pouring, it does not prevent oxidation completely enough to insure castings free from oxide inclusions. Other methods have been proposed and employed for protecting heated'or molten magne'sium from the air by displacing the contactin air with certain such as carbon dioxide and sulphur oxide, or by simply heating and melting in a vacuum; It has been proposed in United States Patent No. 1,540,867 to sprinkle sulphur on the molten metal to provide an atmosphere of sulphur vapor in contact therewith or in United StatesPatent No. 1,720,286 to employsulphur in combination with a flux such as magnesium fluoride in contact with the metal, the flux protecting the metal from the tendency for sulphur to react with the same, while the sulphur above the flux on the metal nerates sulphur vaporto provide a pgotect-ive atmosphere. Gases such as carn dioxide and sulphur dioxide cannot be rehed upon alone to produce a protec 've atmosphere, due to the fact that magnesium will react with these gases according to the .equations':

CO +2Mg=2MgO+C SO,+2Mg=2MgO+S SO +3Mg=2MgO+MgS at temperatures encountered in foundry I tective'gases-owing to their decomposition and formation of carbon when in contact with the heated metal.- Fluxes may be employed advantageously in certain cases for protecting molten metal at temperatures high enough to melt the fluxes so as to give an inert fluid covering over the metal. Fluxes of this type are well known and in general use.

I have found that in some processes of handling heated magnesium and its alloys, either in molten or solid condition, the resence of the aforementioned protective uxes is undesirable. For example, when making oxide, carbon dioxide, or sulphur dioxide,

are in contact with magnesium or its alloys at the temperatures employed in melting, pouring, or casting, some oxide or nitride, or both, is formed, which may contaminate the cast metal.

It has been found especially advantageous in the case of o erating molten baths of such readily oxi izable metals, in connection with die casting machines, to employ gases ratherthan liquid fluxes for protecting the metal from the atmosphere, so as to preclude the occurrence of flux inclusions in the castings.

I am aware that the rare gases of the atmosphere, namely, helium, neon, argon, krypton, or xenon could be employed singly or as mixtures to great advantage as a nonreactive atmosphere for magnesium and its alloys since it is well known that these gases do not form any chemical compounds with other elements. However, the great expense and unavailability of the rare gases above-mentioned preclude their use as protective. atmospheres.

I have discovered a gaseous atmosphere which may be employed in contact with heated magnesium and its alloys at temperatures as high as those encountered in foundry practice, that inhibits the formation of detrimental reaction products and obviates the need for employin fluxes.

My invention is ased upon the discovery of the fact that carbon bisulphide inhibits reactions between magnesium or its alloys, in heated condition, and certaingases, when in contact therewith. Some of the gases on which carbon bisulphide has been found to exert an inhibiting effect on their tendency to combine with, or be decomposed by, these metals at temperatures 'as high as those encountered in foundry practice, include nitrogen, carbon monoxide, carbon .dioxide, sulphur dioxide, or mixtures of these gases. Accordingly, the protective gaseous mixtures which come within the scope of my invention contain a gas normally reactive to carbon bisulphide.

A gas mixture which I have found well adapted to accomplish the protection required in handling heated magnesium and itsalloys maybe generated by treating air according to a process hereinafter described. In treatin air to obtainnitrogen for use in my method of protection I may burn therein any one of the substances sulphur, hydrogen, coal, coke, or the like, so as to remove the oxygen; The products of combustion are then passed through a scrubber to remove substantially all the water, carbon dioxide, or sulphur dioxide, as the case may be, thus leaving a more or less pure nitrogen gas free from oxygen. Carbon bisulphide is added to the nitrogen, prepared as above or otherwise obtained, to form the gaseous mixture to be employed as the protective gas. In some cases it may not be desirable to remove the carbon dioxide or sulphur dioxide in the scrubber since the tendency for-these gases to react with magnesium is inhibited when mixed with carbon bisulphide vapor.

The relative amount of carbon bisulphide in the protective gas mixtures is capable of considerable variation, since compositions containing up to as much as 50 per cent by volume of carbon bisulphide, or more, are effective. Preferably the gaseous mixture may contain from. 1 to 10 per cent by volume of carbon bisulphide when nitrogen is employed, while somewhat larger amounts of carbon bisulphide may be used with the other gases, viz., carbon monoxide, carbon dioxide, sulphur dioxide, or mixtures of these gases.

The manner of the application of my novel protective atmosphere depends upon the nature of the operations involving the metals to be protected. To protect a body of molten magnesium or magnesium base alloy in a melting pot, for example, it is suflicient to provide means to direct the protective atmosphere into contact with the surface of the molten metal so as to prevent thereby the access of air to the same. This may be accomplished by providing the melting pot with a cover having a more or less restricted opening through which molten metal may be added or removed and another opening through which a stream of the protecting gases may be supplied continually,

or in any other convenient manner. Where the container ofthe molten metal is part of a die casting machine, such as the type employing a goose-neck submerged in the body of the moltenmetal, the protective atmosphere may be directed over the surface of the aaoaoeo molten metal, around the goose-neck outlet and through the mold operating in connection therewith so as to provideprotection for both the metal in the melting pot and-in the mold during the filling operation and solidification of metal therein. In operating equipment such as above indicated, a convenient method of determining when a suitable concentration of carbon bisulphide is present in the gaseous mixture may be based tures at which oxide-nitride 7 tinctly claim as my invent on upon maintaining a flow of liquid carbon bisulphide into the gas stream led to the metal surface sufiicient to produce the characteristic'flame ofburning carbon bisulphide vapor at the openings in the equipment where the gas mixture comes in contact with the air. The maintenance ofenough carbon bisulphide in the protective gas mlxtures to give the characteristic flame is an advantage also, when carbon monoxide is employed, as-

it insures the complete burning of this gas to carbon dioxide. a

While I have described the application of the principle of my invention more specifically in connection with a method of protecting magnesium and its alloys in the molten state, such as are encountered in foundry practice, it is to be understood that the method may be employed in connection with the above-mentioned metals in solid state. For example, during heat treatment or other process in which the said metals are heated below the melting point and up to temperafilms are formed, I may advantageously employ my gas mixture containing carbon bisulphide vapor for the working atmosphere instead of air or other gases. 1

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regardsthe process herein'disclosed, rovided the means stated b any of the-fiillowing claims or the equiva ent of such stated means be employed.

I therefore particularly point out and dis- 1. The method -of protecting magnesuim or its alloys while in heated condition, which comprises maintaining in contact wlth .the surface ofthe metala. gaseous mixture of carbon bisulphide vapor and at least one of 1' the gases selected from the group consisting of nitrogen, carbon monoxide, carbon dioxide, sulphur dioxide.

2. The method of protecting magnesium or its alloys while in heated condition, which comprises maintaining in contact with the surface of the metal a gaseous mixture of zarbon bisulphide yapor and nitrogen.

3. The method of protecting magnesium or its alloys while in heated condition, which comprises maintaining in contact with the surface of the metal a gaseous mixture of carbon bisulphide vapor and carbon dioxide.

of magnesium or its alloys, which comprises directing upon the' surface of the molten metal a stream of a gaseous mixture of carbon bisulphide vapor and at least one of the gases selected from the group consisting of nitrogen, carbon monoxide, carbon dioxlde, sulphur dioxide- I 7 A method of protecting a molten bath of magnesium or its allo s, which comprises directing upon the surface of the molten metal a stream ofa gaseous mixture of carbon'bisulphide vapor and nitrogen.

8. A method of protecting a molten bath of ma esium or its allo s, which comprises directing upon the sur ace of the molten metal a stream of a gaseous mixture of carbon bisulphide vapor and carbon dioxide.

7 9. A method f-protecting a molten bath of m esium.or its allo s, which comprises directlng upon the sur ace of the molten 'metal a stream. of a gaseous mixture of carbon bisulphide vapor and sulphur dioxide.

10. A method of protecting magnesmm or its alloys in the solid state at temperatures up to the melting point thereof, which comprises heating the solidmetal in an atmosphere composed of a gaseous mixture of;

carbon bisulphide vapor and at least one of the gases selected from the group consisting of nitrogen, carbon monoxide, carbon dioxide, sulphur dioxide.

11.. A method-of protecting -magnesium or its alloys in. the solid state at temperatnres up to the melting point thereof, which comprises heating the solid metal in an atmosphere composedof a gaseous mixture of carbon bisulphide vapor and nitrogen.

12. A method. of protecting magnesium or its alloys in the solid stateat temperatures up. to the melting point thereof, which comprises heatingthe solid metal in an atmosphere composed of a gaseous mixture of,

carbon bisulphide vapor and carbon dioxide. 13. A met od of protect ng magnesium or its allo sin the solid state at temperatures up to t e melting point thereof, which comprises heating the solid'metal in an atmoshere composed of a gaseous mixture of carbon v bisulphide dioxide. v i 14. As a new product, a gas to be used vapor and sulphur 7 as an inert atmosphere in contact with magnesium audits alloys in heated condition, comprising a mixture of carbon bisulphide and at least one of the ases selected from the group consisting o nitrogen, carbon monoxide, carbon dioxide, sulphur dioxide,

15. As a new roduct, a gas to be used as an inert atmosp ere in contact with magnesium and its alloys in heated condition, comprising a mixture of carbon bisulphide and nitrogen.

16. As a new roduct, a gas to be used as an inert atmosp ere in contact with magnesium and its alloys-in heated condition,

comprising a mixture of carbon bisulphide ang cargo}? dioxiille. d f M h 3 igne y me t is 19 ay o are 19 2.

EDWIN 0. BARsTw. 

